Heredity

Heredity is the passing on of characters (traits) from parents to offspring. The differences among individuals of the same species are called variations. Heredity ensures offspring resemble their parents, while variations make every individual slightly different — and variation is the raw material for evolution.

Traits are controlled by units of inheritance called genes, which are located on chromosomes in the nucleus and are made of DNA. Each parent contributes one set of chromosomes to the offspring through the gametes, so the offspring receives two copies of every gene — one from each parent.

The scientific study of heredity began with Gregor Mendel, who experimented on garden pea plants. He chose pairs of clearly contrasting characters (such as tall vs short plants, round vs wrinkled seeds). When he crossed a pure tall plant with a pure short plant, all the first-generation (F₁) plants were tall — the ‘short’ character had disappeared. When he then self-pollinated these F₁ plants, the next generation (F₂) had both tall and short plants in a ratio of about 3 : 1.

Mendel explained this by saying that a trait can be present but not shown. The character that appears in F₁ (here, tall) is the dominant trait; the one that is hidden (short) is the recessive trait. It reappears in F₂ because each plant carries two factors (genes) for the trait.

Mendel’s work gave us the rules of inheritance. Each trait is controlled by a pair of factors (genes), called alleles. We use capital letters for dominant alleles and small letters for recessive ones — for example, T for tall and t for short.

• A pure tall plant is TT and a pure short plant is tt. Their cross gives all Tt in F₁ — these are tall (because T is dominant) but carry the hidden t. An organism with two identical alleles (TT or tt) is homozygous; one with different alleles (Tt) is heterozygous.
• When two Tt plants are crossed, the F₂ offspring are TT, Tt, Tt, tt — a genotype ratio of 1 : 2 : 1, giving 3 tall : 1 short (the 3 : 1 ratio). This shows that the recessive trait reappears, proving each trait is controlled by a pair of factors that separate during gamete formation.

Sex determination in humans is also controlled by chromosomes. Humans have 23 pairs of chromosomes; one pair is the sex chromosomes. Females have two X chromosomes (XX); males have one X and one Y (XY). All eggs from the mother carry an X. The father’s sperms are of two kinds: half carry X and half carry Y. If an X-carrying sperm fertilises the egg, the child is a girl (XX); if a Y-carrying sperm fertilises it, the child is a boy (XY). So it is the father’s sperm that determines the sex of the child, and there is an equal chance of a boy or a girl.

Evolution is the slow, gradual change in the characteristics of a species over many generations, which has given rise to the great variety of living things on Earth from common ancestors. Variations that can be inherited are the basis of evolution.

An important distinction is between two kinds of traits:

• Inherited traits — present in the genes (DNA) of the reproductive cells, so they are passed to the offspring. Example: eye colour, the shape of a flower.
• Acquired traits — developed during the lifetime of an organism due to its environment or use, and are not present in the reproductive cells, so they are not passed on. Example: the strong muscles of a wrestler, or the low weight of a starved animal. A classic experiment showed that if the tails of mice are cut for many generations, the offspring are still born with tails — because cutting the tail is an acquired trait that does not change the genes.

Evidence for evolution comes from homologous organs (organs with a similar basic structure but different functions, such as the forelimbs of humans, birds and whales — showing a common ancestor), analogous organs (different structure but similar function, such as the wings of a bird and an insect), and fossils (the preserved remains of organisms that lived long ago, which show how life has changed over time). The scientist Charles Darwin proposed the theory of evolution by natural selection — nature selects the organisms best suited to survive and reproduce, so helpful variations become more common over time.